Polyester, in particular a copolymerized polyester containing as a main component, a polyethylene terephthalate (hereinafter, abbreviated as PET) unit or a polybutylene terephthalate (hereinafter, abbreviated as PBT) unit copolymerized with aliphatic dicarboxylic acid or various diols is known. The copolymerized polyester has excellent heat resistance, weather resistance, solvent resistance, flexibility, or the like, and thus is widely used for a film, a fiber, a sheet, various molded articles, and adhesives.
However, when the copolymerized polyester is used for an application in which high flexibility is required, it breaks easily as it has insufficient flexibility at low temperature or room temperature. For such reasons, there is a limit to the use of the copolymerized polyester.
To improve such disadvantages, a method of copolymerizing a polyester resin with a soft segment is suggested. A polyester-polyether block copolymer containing a polyether compound as a soft segment has low glass transition temperature and high fluidity as a resin, and the resin maintains the flexibility even when the molecular weight is lowered. For such reasons, it is widely used as a molding material that is used for electric and electronic components, automotive components, or the like. The polyester-polyether block copolymer is disclosed in Patent Document 1, for example.
However, the polyester-polyether block copolymer is prone to undergo hydrolysis due to the ester bond present in a hard segment. Further, the polyether compound as a soft segment is prone to undergo oxidative degradation, thermal degradation, or the like when exposed to high temperature, and thus the copolymer itself has a problem in wet heat durability or hot moisture resistance.
A polyester resin and a polyester resin composition suitable for molding are disclosed in Patent Document 2. The resin disclosed in Patent Document 2 is suitable for molding use for an electric and electronic component, and it is also described that the resin has excellent water resistance, durability, and fuel resistance.
However, with the composition disclosed in Patent Document 2, flexibility, wet heat durability, adhesiveness, or the like are not obtained at sufficient level. For such reasons, when used for a molding of an electric and electronic component, the product obtained from the resin has a problem that peeling occurs between the resin part and the electric and electronic component present inside or cracks occur at the resin part, and thus it is impossible to use it for a long period of time.
Patent Document 1: Japanese Patent Application Publication No. H2-3429
Patent Document 2: Japanese Patent Application Publication No. H2003-176341